1. Field of the Invention
This invention relates to a gain stabilizing system of an optical amplifier which is designed for amplification of input signal in a rare earth doped optical fiber.
2. Description of the Related Art
Regarding as the technique stated above, the configuration of FIG. 15 is described in the publication "Automatic Gain Control in Cascaded Erbium Doped Fiber Amplifier System", Electronics Letters, Vol. 27, No. 3, pp. 193-195, Jan. 31, 1991.
In FIG. 15, an input signal is entered through an input terminal 1 and input into a rare earth doped fiber 3 via a coupler 4a and a WDM (Wavelength Division Multiplexing) coupler 6. The input signal amplified at the rare earth doped fiber its input into a coupler 4b and output from an output terminal 2. A probe beam source 19 is modulated by an oscillator 20 and input into the rare earth doped fiber through the coupler 4a. Light power of the probe beam source 19 is locking-in detected by an optical receiver 10 connected to a coupler 4b. Supposing that a value of an output signal power of the probe beam source 19 is constant, a gain at the rare earth doped fiber can be known by the optical receiver 10. A pump-source driving circuit 21 controls an output signal power of a pump-source 8 based on the gain detected by the optical receiver 10. The output from the pump-source 8 is input into the rare earth doped fiber through the WDM coupler 6. An optical amplifier whose gain is stabilized can be obtained by operating the pump-source driving circuit 21 so as to make the gain at the rare earth doped fiber, detected by the optical receiver 10, be a constant value. Unused terminals of the coupler 4b and the WDM coupler 6 are reflectionless terminations 5a and 5b.
In the above gain stabilizing system, a probe beam must be used to detect the gain, which is superfluous for the system. Also, a wide dynamic range is needed for controlling the output of the pump-source. However, a signal-to-noise ratio (S/N) of the optical amplifier may deteriorate from changing the output signal power of the pump-source. In addition, since a semiconductor laser is usually used as the pump-source, an oscillation wavelength changes when the output signal power is changed, which enormously deteriorates a characteristic of the optical amplifier. Namely, the range of the input signal has had to be kept narrow.
Configuration of FIG. 16 shows an example of stabilizing the gain without changing the output of the pump-source. This example is reported in the thesis "IEEE Photonics Technology Letters" Vol.3, No. 5, 1991 in pages 453 to 455, entitled "Dynamic Gain Compensation in Saturated Erblum-Doped Fiber Amplifiers" by E. Desurivlre, et al.
In FIG. 16, the input signal enters through the input terminal 1, passes through the coupler 4a and the WDM coupler 6, and is amplified at the rare earth doped fiber 3. After passing the coupler 4b, the signal is output from the output terminal 2. The pump-source 8 is input into the rare earth doped fiber 3 through the WDM coupler 6. A part of the input signal amplified at the rare earth doped fiber 3 and a part of spontaneous emission light amplified at the rare earth doped fiber 3 are separated at the coupler 4b. Only the spontaneous emission light is transmitted through an optical filter 9. The output of the optical filter 9 is detected by the optical receiver 10. An optical modulator 22 is modulated by a compensation-signal controller 24 based upon the output from the optical receiver 10. An output signal from the optical modulator 22 is mixed with the input signal at the coupler 4a.
Since power of amplified-spontaneous-emission (ASE) is dependent on the gain of the optical amplifier, the gain of the optical amplifier can be known by detecting the power of the ASE. The gain of the optical amplifier is changed based on a total power of the input signal and the compensation-signal. Accordingly, the gain of the optical amplifier can be constant by controlling a change of the input signal and a power of the compensation-signal so as to make a detected ASE power constant. In this case, a dynamic range of the pump-source, that is a dynamic range of the gain of the optical amplifier, can be small by using the compensation-signal and no characteristic variation of the optical amplifier occurs. In FIG. 16, a configuration for controlling a general output and a temperature, for the pump-source is omitted.
In addition, it is known that an amplification level is controlled by separating a part of the ASE from an output signal amplified in an optical fiber, and feedbacking the separated ASE to an input side. FIG. 17 shows a configuration of the optical amplifier applying the above system disclosed in Unexamined Japanese Patent Publication Heisei 4-318526. The pump-source 8 and the optical filter 9 which makes some wavelength of the ASE pass selectively, are shown in FIG. 17.
Operation of the optical amplifier is as follows. A part of the ASE and a part of the input signal are taken out in the coupler 4a at an output side of the optical fiber and only the ASE is filtered at the optical filter 9. In this case, supposing that the input signal is decreased, the ASE is increased. The increased ASE from the optical filter 9 is returned to the optical fiber via the coupler 4b, so the amplification level of the optical amplifier is decreased, which keeps the output constant. Thus, a gain decrease can be obtained corresponding to the input decrease in a negative feedback loop.
Since the conventional optical amplifier is constructed as the above, when the dynamic range of the pump-source is extended based on a wide input signal dynamic range, the signal-to-noise ratio is deteriorated and an originated wavelength is changed in the configuration of the optical amplifier shown in FIG. 15. Accordingly, there has been a problem that only a narrow range input signal can be used.
In the configuration of the optical amplifier shown in FIG. 16, it is possible to make the dynamic range of the pump-source narrow even for a wide range input. However, in this case, it is necessary to separate the ASE from the input signal for control and a wavelength of the ASE must be different from that of the input signal. On the other hand, it is preferable for the wavelength of the ASE to be the same as the wavelength of the input signal since the ASE is detected in order to infer the gain. When the wavelength of the ASE and that of the input signal are different for the purpose of separating, control can not be executed correctly. In addition, since the propagating direction of the input signal and that of the compensation-signal are the same, it is difficult to separate the input signal from the compensation-signal and a transmission characteristic of the input signal is deteriorated since the input signal has a non-linear distortion when the compensation-signal is large.
In the configuration shown in FIG. 17, the input signal must be perfectly eliminated by the optical filter because the S/N of the output signal of the optical amplifier deteriorates enormously when the input signal is mixed in a feedback loop. However, it is difficult to perfectly eliminate the input signal, particularly when the wavelength of the input signal widely varies since the transparent wavelength of the optical filter must be changed depending upon the wavelength of the input signal. In addition, there is a possibility of oscillation when the output signal is used as a feedback source and input into the optical amplifier.
The present invention is for solving the above problems. It is an object of this invention to obtain a correct amplification level by using a pump-source and a compensation-signal source whose wavelengths are almost the same length as that of an input signal, even when the range of the input signal is wide, and to obtain an optical amplifier whose characteristic does not deteriorate. It is another object to obtain a system in which the optical amplifier is applied.